Immunological Functions of the Alimentary Tract

• What are the two main divisions of the human immune system?

  • Systemic immunity: bone marrow, spleen, thymus, lymph system, and blood circulation.
  • Mucosal immunity: mucous membranes (e.g. eyes, nose, mouth, lungs, gut, genitourinary tract).
• What are the key differences between innate and adaptive immunity?

  • Innate immunity: Non-specific, no memory, uses barriers and cells like macrophages.
  • Adaptive immunity: Specific, has memory, involves lymphocytes and antibodies.
• Why is the mucosal immune system important in the alimentary tract?

  • Major site for microbial entry
  • Specialized for absorption
  • Must distinguish pathogens from food and commensals
  • Surface area: gut ≈ 200 m² vs skin ≈ 2 m²
• Name four innate protective mechanisms of the mucosal immune system.

  1. Mucin
  2. Peristalsis
  3. Antimicrobial proteins (e.g. lysozyme, lactoferrin)
  4. Phagocytes
• What immunoglobulins are primarily involved in mucosal immunity?

  • Secretory IgA (SIgA) and IgM
  • Some involvement of IgG
• What are the primary lymphoid structures in the gut?

  • Intraepithelial lymphocytes
  • Lymphocytes and macrophages in lamina propria
  • Peyer's patches
• What are M cells and their function in mucosal immunity?

  • Specialised epithelial cells in Peyer’s patches
  • Transport antigens and microbes from the gut lumen to immune cells
  • Important for initiating immune responses
• What types of pathogens do M cells transport?

  • Particles: cholera toxin, latex, ferritin
  • Viruses: poliovirus, HIV
  • Parasites: Cryptosporidium
  • Bacteria: Cholera, Salmonella, Shigella, E. coli, etc.
• What is the "common mucosal immune system"?

  Immune cells activated in one mucosal site can migrate and function at others (e.g. gut → lung or mammary gland)
• What are the key functions of SIgA in mucosal immunity?

  • Neutralises toxins/viruses
  • Agglutinates pathogens
  • Prevents pathogen attachment (immune exclusion)
  • Interacts with lysozyme, lactoferrin
  • Excreted into mucosal secretions
• How does oral vaccination compare to systemic vaccination?

  • Oral: Induces mucosal (SIgA) response
  • Systemic: Induces strong IgG response but limited mucosal immunity
  • Most vaccines today are systemic, despite mucosal entry points of pathogens
• What is oral tolerance?

  • A phenomenon where oral antigens suppress systemic immune responses
  • Helps prevent overreaction to food and commensal microbes
• What factors influence the induction of oral tolerance vs. vaccination?

  • Tolerance: Soluble antigens, high repeated doses
  • Vaccination: Formulated antigens (e.g. with adjuvants), limited doses
• Why are GM plants studied for oral vaccines?

  • Can express antigens (e.g. Hep B) in edible tissues
  • Enables oral delivery of vaccines, potentially inducing both systemic and mucosal responses
• What are the structural components of the mucosal barrier?

  • Tight junctions between epithelial cells
  • Mucus layer (produced by goblet cells)
  • Digestive enzymes (e.g. proteases, lysozyme)
  • Peristalsis
  • Secretory immunoglobulins (SIgA, SIgM)
• What are the key cell types found in Peyer’s patches?

  • M cells
  • B lymphocytes
  • T lymphocytes
  • Dendritic cells
  • Macrophages
  • Follicle-associated epithelial cells
• What are the functions of goblet cells and Paneth cells in the gut?

  • Goblet cells: secrete mucus to form a protective barrier
  • Paneth cells: secrete antimicrobial peptides (e.g. defensins) to kill pathogens
•  What immune cells are found in the lamina propria?

  • T cells
  • B cells
  • Macrophages
  • Dendritic cells
  • Plasma cells
• What are the limitations of oral vaccines?

  • Antigen degradation by stomach acid and enzymes
  • Difficulty in crossing the mucosal barrier
  • Risk of inducing oral tolerance instead of immunity
  • Requires protective delivery (e.g. liposomes, capsules, adjuvants)
• What are some strategies to enhance oral vaccine delivery?

  • Attenuated bacteria or viruses (e.g. Salmonella typhi)
  • Mucosal adjuvants (e.g. cholera toxin)
  • Liposomes or microspheres
  • Genetically modified (GM) edible plants
• Why is COVID-19 a challenge for mucosal immunity?

  • COVID-19 is a mucosal infection (affects respiratory tract)
  • Current vaccines are systemic (injected), so they provide limited mucosal protection
  • Future strategies may include nasal or oral vaccines
• How might oral tolerance be used in treating disease?

  • Oral administration of antigens may suppress immune responses
  • Possible treatment strategy for autoimmune diseases like MS or type 1 diabetes
  • Still under investigation
• What determines whether oral antigen exposure leads to tolerance or immunity?

  • Tolerance: Soluble antigens, high/repeated doses, no adjuvants
  • Immunity: Use of adjuvants or formulations, low/intermittent dosing
• What are key differences between serum IgG and secretory IgA?

  IgG:

  • Location: Serum
  • Function: Inflammation, opsonisation
  • Structure: Monomer
  • Inflammation: Yes

  SIgA:

  • Location: Mucosal secretions
  • Function: Immune exclusion, neutralisation
  • Structure: Dimer with secretory component
  • Inflammation: Minimal